The present invention relates to a device for detecting torque applied to a rotary shaft to be rotated, and more particularly to a torque detector to be mounted on a car wherein the torque applied to the rotary shaft such as an output shaft of a transmission of the car is found by a quantity of torsion of the rotary shaft.
Recently, there has been increased the necessity of torque detection in the car. Referring to an automatic transmission using a torque converter, particularly, feedback control is executed by using the torque of the output shaft, which is precisely detected, so that the characteristics of the automatic transmission can greatly be enhanced. Referring to a four wheel drive, it is very important to detect torque distribution between front and rear wheels.
When the torque is applied to the rotary shaft, the torsional deformation, which is proportional to the torque, occurs on the rotary shaft. When the applied torque is represented by T, a torsional angle .theta. of the rotary shaft is found by the following formula (1). The torsional angle .theta. is measured between two positions which are separated from each other by a distance L in the axial direction of the rotary shaft; EQU .theta.=64TL/2.pi.Gd.sup.4 ( 1)
where d is a diameter of the rotary shaft, and G is a transverse elastic modulus which is determined by the material of the rotary shaft. The formula (1) is transformed as follows. EQU T=2.pi..theta.Gd.sup.4 /64L (2)
If the diameter d and the transverse elastic modulus G of the rotary shaft are known values, the applied torque T can be calculated with the formula (2) by using the torsional angle .theta. of the rotary shaft measured between the two positions which are separated from each other by a distance L in the axial direction of the rotary shaft.
There have been proposed various torque detectors in accordance with the above-mentioned principle. There has been known a torque detector using two optical rotary encoders. Japanese Unexamined Patent Publication No. 62-239031 has disclosed a torque detector using two magnetic rotary encoders comprising magnetic drums or magnetic disks as magnetic scales and non-movable magnetic sensors. The magnetic scales are individually and coaxially fixed to the rotary shaft in two positions separated from each other by a distance L in the axial direction of the rotary shaft. The non-movable magnetic sensors are provided opposite to the magnetic faces of the magnetic scales. Each of the magnetic scales is premagnetized such that a lot of magnetic poles are arranged at a constant pitch over the entire periphery of the magnetic face before it is fixed to the rotary shaft. Each of the magnetic sensors includes a magnetoresistive effect element for converting the change of magnetic flux into a cyclic electric signal and outputting the same. The change of magnetic flux is caused by the magnetizing patterns on the magnetic faces of the magnetic scales while the rotary shaft is rotated. Two sets of magnetic scales and magnetic sensors form two magnetic rotary encoders for measuring the torsional angle .theta. of the rotary shaft.
When the torsional deformation occurs on the rotary shaft owing to the torque, phase difference, which is proportional to a quantity of torsion of the rotary shaft, is caused between the output signals of the two magnetic sensors. With the following formula (3) can be found the relationship between the torsional angle .theta. of the rotary shaft and the phase difference .DELTA.t measured in a unit of time between the output signals of the magnetic sensors; EQU .theta.=2.pi..DELTA.tN (3)
where N is the number of rotations for a unit time of the rotary shaft. The number of rotations can be obtained with the following formula (4) by using the measured values of frequencies f of the output signals of the magnetic sensors; EQU N=f/U (4)
where U is the number of magnetic poles on the magnetic scales and a known invariant. If the torsional angle .theta. of the rotary shaft is found with the formulas (3) and (4), the applied torque T can be obtained with the formula (2).
Referring to the conventional torque detector using two magnetic rotary encoders, however, there is a fundamental problem that the torque cannot be detected with high precision. A first factor of preventing the torque from being detected with high precision is that a circumferential positional shift is almost inevitably caused between the magnetic scales when they are fixed to the rotary shaft, or the circumferential positional shift is almost inevitably caused between the magnetic sensors when they are provided opposite to the magnetic scales respectively. While the rotary shaft is released from a load and is rotated with no torque, the offset of the phase difference is caused between the output signals of the magnetic sensors owing to the positional shift of the magnetic scales or magnetic sensors so that apparent torque is detected with no torsional deformation of the rotary shaft. In other words, the offset occurs on the detected torque. A second factor is the distortion which remains on the rotary shaft while the rotary shaft is loaded. When the distortion remains on the rotary shaft itself, the offset of the phase difference is caused between the output signals of the magnetic sensors owing to the residual distortion even if no torque is applied to the rotary shaft. Consequently, the offset occurs on the detected torque.
To eliminate the offset of the detected torque, a novel torque detector has been disclosed in Japanese Patent Application No. 1-254561 (Sep. 29, 1989) which was laid open as Japanese Unexamined Patent Publication No. 3-115940 (May 16, 1991). The torque detector comprises magnetic disks and non-movable magnetic heads for recording and reproducing. The magnetic disks are individually and coaxially fixed to the rotary shaft in two positions separated from each other by a distance L in the axial direction of the rotary shaft, and is rotated together with the rotary shaft. The non-movable magnetic heads are provided opposite to the magnetic faces of the magnetic disks.
Referring to the torque detector disclosed in the above-mentioned application, respective cyclic in-phase electric signals are provided as recording signals to the two magnetic heads while the rotary shaft is released from a load and is rotated with no torque in a preparatory step prior to the detection of the applied torque. The magnetic heads execute recording operation for converting the respective recording signals into the respective corresponding change of magnetic flux, i.e., magnetic signals. As a result, in-phase magnetic signals of the magnetic heads are recorded in the form of respective magnetizing patterns on the magnetic faces of the magnetic disks such that a lot of magnetic poles are arranged at a predetermined pitch over the entire periphery of the magnetic disks. Thus, two magnetic disks are respectively fixed to the rotary shaft and then the same magnetizing pattern for torque detection is simultaneously formed on the respective magnetic faces of the magnetic disks. Differently from the torque detector using two magnetic rotary encoders, therefore, it is not necessary to consider the circumferential positional shift between the magnetic disks when they are respectively fixed to the rotary shaft.
The magnetic heads also execute the reproducing operation for reading out the respective magnetizing patterns on the magnetic faces of the magnetic disks. Each magnetic head has the function of converting the change of magnetic flux into a cyclic electric signal and outputting the same. The change of magnetic flux is simultaneously caused by the magnetizing patterns on the magnetic faces while the rotary shaft is rotated. Each magnetic head is held in the same position at the time of reproducing and recording. Differently from the torque detector using two magnetic rotary encoders, therefore, the influence of the positional shift of the magnetic heads is set off between the recording and reproducing operation even if the circumferential positional shift of the magnetic disks is caused between the magnetic heads when they are provided opposite to the magnetic faces of the magnetic disks. In other words, the positional shift of the magnetic heads does not cause the offset of the phase difference between the output signals of the magnetic heads. In the case where the offset of the phase difference is caused between the output signals of the magnetic heads owing to the residual distortion of the rotary shaft with no torque applied thereto, the operation of the preparatory step is executed again to reform the magnetizing patterns on the magnetic faces of the magnetic disks. Consequently, the offset of the phase difference can be canceled between the output signals of the magnetic heads.
When the torsional deformation occurs on the rotary shaft owing to the torque, the phase difference, which is proportional to the quantity of torsion of the rotary shaft, is caused between the output signals of the magnetic heads. A torsional angle .theta. of the rotary shaft is found with the formula (3) based on the phase difference .DELTA.t measured in a unit of time between the output signals of the magnetic heads. Differently from the torque detector using two magnetic rotary encoders, the number of magnetic poles formed on the magnetic faces of the magnetic disks is changed correspondingly to the number of rotations N.sub.0 for a unit time of a non-load rotary shaft in the preparatory step and frequencies f.sub.0 of the in-phase recording signals provided to the magnetic heads in the preparatory step. Consequently, it is necessary to obtain the number of rotations N for a unit time of the rotary shaft loaded in a torque detection step with the following formula (5) in place of the formula (4); EQU N=(f/f.sub.0)N.sub.0 ( 5)
where f is a measured value of the frequency of the output signal of either of the magnetic heads in the torque detection step. In the torque detection step, there are known the number of rotations N.sub.0 of the rotary shaft in the preparatory step and the frequencies f.sub.0 of the in-phase recording signals provided to the magnetic heads in the preparatory step. When the torsional angle .theta. of the rotary shaft is found with the formulas (3) and (5), the applied torque T can be obtained with the formula (2) in similar to the foregoing.
However, the torque detector using two sets of magnetic disks and magnetic heads has a problem to be solved. The problem is caused by the rotary shaft which is not always rotated at a constant speed. In general, the rotary shaft carried on a power transmission system of a car frequently changes its rotary speed. In the case where the number of rotations N for a unit time of the rotary shaft in the torque detection step is to be determined with the formula (5) and the number of rotations N.sub.0 for a unit time of the rotary shaft in the preparatory step is actually varied, the pitch between the magnetic poles formed on the magnetic faces of the magnetic disks is varied even if the recording signals respectively having constant frequencies f.sub.0 are provided to the magnetic heads in the preparatory step. The variation of the pitch between the magnetic poles overlaps that of the rotary speed of the rotary shaft in the torque detection step. Consequently, the frequencies f of the output signals of the magnetic heads in the torque detection step are greatly varied. Thus, the error of the number of rotations N for a unit time, which is obtained with the formula (5), is increased so that the error of the detected torque T is made greater.
It is an object of the present invention to provide a torque detector for detecting torque applied to a rotary shaft to be rotated wherein the influence of the positional shift of components of the torque detector and the influence of the residual distortion of the rotary shaft are eliminated and the influence of the variation of the rotary speed of the rotary shaft is reduced so that the torque can be detected with high precision. It is another object of the present invention to detect with high precision torque applied to a rotary shaft in a power transmission system of a car comprising a plurality of rotary shafts which are removably connected to one another.